In a computer network, NAS (Network Attached Storage) file servers provide file services for clients connected in a computer network using networking protocols like CIFS or any other stateful protocol (e.g., NFS-v4). Usually, when a file, directory, or a server share is migrated from one server to another, the administrator takes the server offline, copies the files to the destination server, and finally brings the destination server online. The larger the amount of data been migrated, the longer the clients must wait for the migration to complete, which leads to longer server down-time.
In today's information age of exponentially growing server capacity and clients spread all over the globe, the amount of down-time an administrator can afford is constantly shrinking. It becomes almost impossible to migrate files from one server to another. This forces storage administrators to buy servers with significantly greater capacity (i.e., overprovision) in order to avoid/delay the need of migrating server data to a newer, higher capacity model.
A common approach to migrate files is to start migrating files while the source server is continued to be accessed and gradually copy all files to the destination server. On the subsequent passes only the newly modified files and directories (since the last pass) are copied and so on. This process is repeated until all files are migrated to the destination server. At this point, the source server is taken offline and replaced with the destination server, thus lowering the amount of time needed to migrate from one server to another. Although this solution lowers the down time it does not completely solve the problem with files that are constantly accessed or held open in exclusive mode. For those files, the user still suffers a visible access interruption and will have to invalidate all of its open handles and suffer service interruption during the migration of those files.